Flyback IC not oscillating transformer primary coil so no voltage on secondary

MisterBill2

Joined Jan 23, 2018
19,044
In a tuned resonant circuit the inductance matters a great deal, since it is a major factor in setting the frequency. In a feedback oscillator the turns ratio matters because usually that is where the feedback comes from. In this circuit the feedback loop is not quite so obvious, although it must be from the "bias winding", terminals 11 and 12. If the phase of that winding is backward then there would not be any oscillations.
I had not realized that the transformer was not the specified device. That alters the picture a bit.
 

AlbertHall

Joined Jun 4, 2014
12,385
In a tuned resonant circuit the inductance matters a great deal, since it is a major factor in setting the frequency. In a feedback oscillator the turns ratio matters because usually that is where the feedback comes from. In this circuit the feedback loop is not quite so obvious, although it must be from the "bias winding", terminals 11 and 12. If the phase of that winding is backward then there would not be any oscillations.
I had not realized that the transformer was not the specified device. That alters the picture a bit.
These devices have a built-in oscillator. They do not rely on the transformer for oscillation.
 

MisterBill2

Joined Jan 23, 2018
19,044
These devices have a built-in oscillator. They do not rely on the transformer for oscillation.
OK, and I see that indeed there is nothing approaching a resonant circuit at all. So what does set the frequency?? And does any part of the external circuit have any effect on the frequency?
If not, then the only conclusion is poor connections or possibly wrong connections.
Those solderless breadboards are seldom suitable for switching power supplies of any kind.
 

shortbus

Joined Sep 30, 2009
10,049
Aswellas the fact that for the TOP257 only reccomends the bobbin and the application notes tell you how to design the transformer where as I want to use an off the shelf component.
I suggest that you down load the data sheet for your transformer and the one in my link. From the short look at both, that I did, they are only alike because they are called transformers. Since this is YOUR project I only wanted to point you in the right direction, I'm too busy to do the design for you.

For somethings transformers are kind of interchangeable, but for many more they are not, and your project is one of the ones they seem not to be interchangeable. If they were the Application Note would have a list of those that will work.
 

Thread Starter

jlawley1969

Joined Feb 22, 2021
97
I suggest that you down load the data sheet for your transformer and the one in my link. From the short look at both, that I did, they are only alike because they are called transformers. Since this is YOUR project I only wanted to point you in the right direction, I'm too busy to do the design for you.

For somethings transformers are kind of interchangeable, but for many more they are not, and your project is one of the ones they seem not to be interchangeable. If they were the Application Note would have a list of those that will work.
back in this thread...
Ive got the circuit to work I believe it was an issue with cross talk involving the optoisolator/control pin because I am on a breadboard.
the issue cannot seem to get rid of is the fact that I am now getting 700V floating. It is almost like the Zener isn't even doing anything to clamp it but it tests fine.

side note I have these solderable breadboards would that be more advantageous than the solderless variety?
 

shortbus

Joined Sep 30, 2009
10,049
side note I have these solderable breadboards would that be more advantageous than the solderless variety?
Either that or a Vero type board. No matter which you chose follow what they said in your app note about keeping connections close cropped. Long wire joining things together on a complicated circuit like this have consequences, they promote noise, added inductance and other problems to the circuit.
 
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